Satellite image of Earl from Tuesday morning. (NASA)

On Tuesday morning, an Air Force Hurricane Hunter flew into the tropical wave zipping through the Caribbean Sea and found evidence that it has reached tropical storm status.

The tropical storm, named Earl, is the fifth named storm of the season, and the southernmost. It is the earliest storm to develop starting with the letter “E” since Ernesto in 2012 (Ernesto was named a few hours sooner on Aug. 2).

The governments of Mexico and Belize have posted tropical storm warnings for the east coast of the Yucatan Peninsula (except the northern tip), while the government of Honduras has issued a warning for its entire north coast.


(National Hurricane Center)

Conditions are favorable for Earl to intensify.

The ocean temperature in the western Caribbean is in the range of 29 to 30 degree Celsius (84 to 86 degree Fahrenheit), which is like a jolt of caffeine for tropical cyclones! The vertical shear, which can be disruptive to storm development, is moderate but is forecast to soon relax. However, the window for intensification is short because it has only another day or so until it makes landfall.

The center should reach Belize or the Yucatan on Wednesday afternoon or  evening, most probably as a moderate tropical storm. After crossing the Yucatan Peninsula, it is expected to continue tracking toward Mexico’s Gulf Coast.


Two-day track forecasts from a variety of model runs on Tuesday morning. (UAlbany)

Before Earl was named, there was controversy as to why it wasn’t designated a tropical storm sooner. The government of Jamaica took the unusual step of issuing a tropical storm warning before the storm was initially named.

“[I]n my 32 years of Caribbean forecasting, I’ve never seen it before,” said John Morales, chief meteorologist for the NBC affiliate in Miami.

Fast-moving tropical waves sometimes take longer to be named tropical storms than others. By definition, a tropical storm must have a “closed surface wind circulation about a well-defined center,” among other things. But rapid forward speed can impede an open wave from attaining a closed circulation.

In the cartoon below, the left side shows a weak vortex with 25 mph winds rotating around it. This is how a storm would appear if it were stationary or viewed in a “storm-relative” reference frame. But if we add motion to it — westward at 20 mph, for example — the “Earth-relative” easterly winds are stronger on the north side and westerly winds on the south side are barely sufficient to make it a closed circulation.


Schematic showing wind speed around a stationary vortex (left) and the same vortex moving to the west (right).

The dilemma hurricane forecasters are faced with is: Do they issue a tropical storm warning when a tropical weather system may have the hazardous characteristics of a tropical storm but not meet its strict criteria?